In a spark plug in which a center electrode is held in an insulated manner via an insulator by a metal shell, when carbon generated by incomplete combustion or the like is deposited on a surface of a front end portion of the insulator, decreasing insulation resistance, and an applied voltage is less than a required voltage (voltage that causes spark discharge), a spark discharge does not occur. In a spark plug having high heat resistance (higher heat rating), the insulator nose length (front end portion) is short, and a creepage distance, of the front end portion, from the metal shell to the center electrode is short. Therefore, there is a concern of a reduction in insulation property due to the deposit of carbon. On the other hand, in a spark plug having low heat resistance (lower heat rating), the insulator nose length is long, thus improving insulation property. However, there is a concern that heat transfer performance is not good and heat resistance is decreased.
For example, Japanese Patent Application Laid-Open (kokai) No. H6-176848 discloses a technique in which in a spark plug having low heat resistance (length of a front end portion is 17 mm), grooves opened to the radially outer side are provided at a relatively thick portion of the insulator nose length (front end portion). In this case, since carbon adhered to the front end portion can be made discontinuous in the axial direction by the grooves, it is possible to suppress a reduction in insulation property due to the deposit of carbon.
However, in the above conventional technique, there is a need for ensuring insulation property and improving heat resistance. In a spark plug having a higher heat rating, if grooves opened to a radially outer side are provided at a front end portion with reference to the technique of Japanese Patent Application Laid-Open (kokai) No. H6-176848 that is a technique of a spark plug having a lower heat rating, the strength of the insulator might be reduced. Therefore, it is impossible to ensure a sufficient creepage distance.
The present invention has been made in order to meet the aforementioned need. An advantage of the present invention is a spark plug that can realize both heat resistance and insulation property.
According to a first aspect of the present invention, there is provided a spark plug having a center electrode having a nose portion that extends from a front side to a rear side along an axial line, and a flange portion that projects to a radially outer side from a rear end of the nose portion. In an annular insulator, a receiving portion supporting the flange portion is formed on an axial hole formed along the axial line, and a step portion having a diameter that increases from the front side toward the rear side is formed on an outer circumferential surface. In a cylindrical metal shell disposed at the radially outer side of the insulator, a shelf portion supporting the step portion via a packing is formed on an inner circumferential surface. A front end portion of the insulator present on the front side with respect to a front end edge of a first contact surface that the packing contacts with, of the step portion, has a length L of less than or equal to 9 mm in an axial direction. Therefore, it is possible to shorten a heat dissipation path. Accordingly, it is possible to improve heat resistance of the spark plug.
In the front end portion, an annular groove opened to the front side is formed around the axial line. The groove has a width of greater than or equal to 0.2 mm in the radial direction. In a cross section including the axial line, a value D/L obtained by dividing, by the length L, a creepage distance D, from a position P on the frontmost side of a region in which a clearance distance between an outer surface of the front end portion and the inner circumferential surface of the metal shell is less than or equal to 0.1 mm to a connection position between the outer surface of the front end portion and the axial hole, is greater than or equal to 1.1. Since the annular groove is opened to the front side, it is possible to form a groove on a relatively thick portion separated by a predetermined distance from the front end of the insulator. Therefore, without reducing the strength of the insulator, it is possible to enlarge the ratio of the creepage distance of the front end portion to the length L. Since it is possible to suppress a decrease in insulation resistance of the front end portion to which carbon adheres, an effect of both realizing heat resistance and insulation property can be obtained.
According to a second aspect of the present invention, there is provided a spark plug as described above, wherein a position in the axial direction of a front end edge of a second contact surface that the flange portion contacts with, of the receiving portion, is the same as the position of the front end edge of the first contact surface or is located on the front side with respect to the front end edge of the first contact surface. Heat at a center side of the front end portion of the insulator can be mainly dissipated from the center electrode, and heat at an outer side of the front end portion can be mainly dissipated from the metal shell. Since it is possible to smoothly dissipate heat from a plurality of heat dissipation paths, not only an effect of claim 1 but also an effect of improving heat resistance can be obtained.
According to a third aspect of the present invention, there is provided a spark plug as described above, wherein a radially outside portion with respect to the groove, of the front end portion, has a front end surface at a position in the axial direction within 2 mm toward the front side from the position P. Since it is possible to shorten a heat dissipation path of a portion at the radially outer side portion with respect to the groove, of the front end portion, not only the effect of claim 1 or 2 but also an effect of improving heat resistance can be obtained.
According to a fourth aspect of the present invention, there is provided a spark plug as described above, wherein the insulator includes an annularly formed first member, and an annularly formed second member disposed at the radially outer side of the first member. The receiving portion is formed on the inner circumferential surface of the first member, and the step portion is formed on the outer circumferential surface of the second member. Since the groove is formed by a gap between the outer circumferential surface of the first member and the inner circumferential surface of the second member, it is possible to easily form the groove. As a result, not only the effect of one of claims 1 to 3 but also an effect of easily manufacturing a spark plug that can realize both heat resistance and insulation property can be obtained.